The present embodiments relate to a magnetic resonance coil for transmitting and/or receiving magnetic resonance signals.
Magnetic resonance coils are used in magnetic resonance devices as transmission coils for transmitting magnetic resonance signals that deflect the nuclear spins and/or as a receiving coil for receiving magnetic resonance signals, from which the magnetic resonance images may be determined.
The use of local coils (e.g., antenna systems that are arranged immediately on (anterior) or beneath (posterior) an object to be examined such as a patient) is known. Local coils may be used to record magnetic resonance images with a high signal-to-noise ratio (SNR). The voltages that are induced by excited cores in the individual coil elements of the local coil during magnetic resonance measurement may be amplified with a low-noise pre-amplifier (e.g., low noise amplifier, LNA) and passed on (e.g., in a cable-bound manner) to the receiving electronics. In order to further improve the signal-to-noise ratio (e.g., in high-resolution images), high-field systems that may have a basic field strength of 1.5 Tesla to 12 Tesla and more are used.
The main advantage of the local coils is that the very small coil elements close to the object allow a very high signal-to-noise ratio. For this reason and on account of the possibility of accelerated measurement by k-space undersampling (e.g., parallel imaging), there is a great deal of interest in very tight arrays of coil elements and therefore local coils with a high number of read-out channels.
In these arrays of coil elements (e.g., antenna arrays), the coil elements are intended to be decoupled from one another as effectively as possible. The prior art discloses different possible ways of achieving decoupling (e.g., inductive decoupling using a carrier, capacitive decoupling using a common coil conductor, and geometric inductive decoupling, since there is an overlap between adjacent coil elements). Intersection regions that contain geometrically decoupled coil elements are produced in each local coil. The conductor tracks (e.g., coil conductors) of the coil elements intersect in the intersection regions. Parasitic capacitances may be produced, and therefore, dielectric losses may occur.
In the case of extremely tightly packed arrays of coil elements (e.g., in local head coils with 32 or more channels and therefore coil elements; in shoulder coils starting from 10 channels), the coil elements are very small, and loss mechanisms in the intersection regions gain increasing influence with respect to image quality. This analogously applies to other arrays of coil elements, in which the individual coil elements are to be very small for other reasons (e.g., in the case of magnetic resonance coils for taking images of animals and in chemical applications).
The mentioned loss mechanisms in the intersection regions are, firstly, resistive losses due to eddy currents that are generated by the conductor of a coil element in the conductor of the other coil element. Secondly, the loss mechanisms in the intersection regions are also dielectric losses due to the parasitic capacitance that is produced in the intersection region. When cost-effective dielectrics are used (e.g., as supports for the coil conductors), the losses at these points may be particularly noticeable, since these materials may also have a high loss factor (tan δ).
In addition to the losses due to dielectric and resistive losses, the coupling of the coil elements at the intersection points may also produce undesired modes (e.g., common modes) that may propagate over the entire array of coil elements. In the case of reception, this may lead to signal losses, and in the case of transmission, this may lead to undesired resonance of the local coil and as a result, to distortion of the B1 homogeneity (e.g., the homogeneity of the high-frequency transmission field).
In order to solve this problem, the coil conductors may be tapered at the intersection points, and the coil conductors may be configured to be narrower, so that the parasitic capacitance falls. However, this may be counter-productive in coil elements, since wider coil conductors lead, overall, to relatively low losses.
It has also been proposed to provide manually soldered bridges in the intersection regions, so that there is a greater distance between the coil conductors of the various coil elements and, as a dielectric, there is also air between the coil conductors. Air has very good loss properties (e.g., with respect to the loss factor). However, this is disadvantageous because the manual work is costly and time-consuming and creates results that are difficult to reproduce.